In the art of demodulation of phase or frequency modulated signals, typical approaches require the determination of an arctangent of a particular magnitude, obtained as a ratio of two signals which may be represented in analog or digital form. One example of this technique is found in U.S. Pat. No. 4,090,145 to Webb.
Webb describes a demodulator in which received input data are sampled by an analog-to-digital quadrature sampler, under control of a timing device, to provide a sequence of digital samples which are multiplied by quadrature digital signals. The result of the process is a pair of quadrature voltage samples, E.sub.1 (t) and E.sub.2 (t), respectively representing cosine and sine components of the input signal. As is known in the art, the modulating angle of the input signal is thereafter obtained by determining the arctangent of the ratio E.sub.1 (t)/E.sub.2 (t).
The Webb patent discloses a digital circuit for determining the modulating phase angle, specifically illustrating in one embodiment a divider circuit to obtain the ratio and an arctangent calculator. It is well known that division is a time consuming operation. The Webb patent attempts to overcome the speed problem by providing a digital divider which requires the use of ROMs (Read Only Memories) for storing log tables in order to perform the division. Additionally, the Webb demodulator requires the use of a further ROM in a table-lookup procedure for determining the arctangent of the computed ratio.
Since the arctangent table, alone, does not identify the actual angle but can only identify an angle within a 45 degree range, an additional circuit is required in order to compute the specific octant in which the angle lies and thus finally to thereby determine the modulating angle.
However, the result does not yet provide the modulating frequency. Thus, where the signal of interest is frequency modulated, the prior art requires provision of a differentiator in order to determine the modulating frequency. As is known, differentiators are subject to significant errors due to transient noise signals.
U.S. Pat. No. 4,317,210 to Dekker et al similarly discloses a demodulator in which an arctangent computer and a differentiator are required.
Thus, the prior art provides a slow, cumbersome approach to frequency demodulation, subject to errors, requiring the use of complex division circuitry and necessitating incorporation of a table lookup technique and a differentiation process to obtain the modulating frequency.
Prior art analog demodulators are known, as represented by U.S. Pat. No. 3,045,180 to Losher. Losher obtains in-phase and quadrature signal samples and requires a conversion from rectangular to polar coordinates. Such demodulators, however, provide less accurate results in view of their susceptibility to variations in component tolerances, temperature drift problems, and the like.
It can thus be seen that there is a need for an efficient digital phase demodulator which avoids the use of division and arctangent computation, eliminates the requirement of a table lookup procedure to determine the arctangent, provides an output directly indicating the modulating angle and quadrant, and determines the modulating frequency without necessity for differentiation.